The present invention relates to incandescent lights, and in particular to lights having light directing means on its envelope.
The emphasis on energy conservation has often focused on increasing the efficiency of lighting devices. Recent efforts to increase the efficiency of incandescent lights have had limited success.
Conventional incandescent light bulbs employ a glowing electrical filament inside of an evacuated glass envelope. Such light bulbs have been manufactured with transparent glass envelopes which make the incandescent filament visible. A disadvantage with this type of filament is the appearance of a very bright point source of light. This point source can cause glare when the light is used in a lamp fixture that does not have an effective diffuser.
To avoid the glare of a hot spot, known glass envelopes of incandescent lights have been "frosted" or etched to make the bulb surface itself a diffuser. Thus, the bulb surface appears as a diffuse source propagating light from the entire surface of the bulb. Because the light is thus diffused, bright spots and glare are reduced. Unfortunately, diffused light lacks the advantages of collimated light, which is sometimes helpful when examining objects or reading. For example, shadows cast from diffuse light are often blurry. When examining small three dimensional objects, sharp shadows can help reveal details of the object under examination. Accordingly, there is a need to reach a balance between diffused and collimated light.
Known spotlights have used an envelope with a parabolic reflector adjacent a glass window. For example, in U.S. Pat. No. 4,728,848 the glass window may be a Fresnel lens designed to focus the radiating light into a collimated beam. Spotlights of this type are inappropriate for illuminating a room. Were such a light installed in a conventional table lamp a focused beam would project upwardly to the ceiling but would not effectively illuminate the room.
Known light fixtures employ Fresnel lenses. These lenses help focus the light and increase the efficiency by directing light in a preferred direction. Again however, these fixtures are inappropriate for common applications such as table lamps. See for example U.S. Pat. Nos. 3,654,451 and 4,520,436.
In U.S. Pat. No. 4,734,836 an accessory mounted directly onto the envelope of a fluorescent tube employs a Fresnel lens grating. Again this device is designed to provide light flux in a preferred direction and is not effective for increasing the apparent efficiency for an incandescent room light.
See also U.S. Pat. No. 4,703,403 where a series of prisms encircle a cylindrical cover for an electric light. See also U.S. Pat. Nos. 2,782,297; 3,221,162; 3,662,208; 3,761,957; 4,160,929; 4,210,841; 4,300,068; 4,315,186; 4,517,491; 4,517,630; 4,530,040; 4,577,260; 4,652,979; 4,690,141; 4,965,488.
Accordingly, there is a need for an improved incandescent light that has a simple and effective means for enhancing efficiency.